Biodegradable polymers useful for implantation into tissue defects and providing scaffolding for tissue ingrowth have been described, for example, in PCT publication WO 9315694, incorporated herein by reference. Such polymers may be manufactured to have mechanical properties matching those of the tissue into which they are to be implanted.
Implants formed of biodegradable polymeric materials may be preseeded with cells of the desired tissue type, for example as described in U.S. Pat. Nos. 4,963,489, 5,032,508, 5,160,490 and 5,041,138.
Implant materials as described above generally contain pores and/or channels into which the tissue grows as the biodegradable material erodes, thus providing new tissue growth of roughly the same size and shape as the implant.
A shapable implant material useful for a dental implant is described in U.S. Pat. No. 3,919,773 to Freeman (Sybron Corp.) issued Nov. 18, 1975 for "Direct Moldable Implant Material," however this implant material is not biodegradable.
PCT publication WO 92/15340 dated 17 Sep., 1992 of Lundgren et al. (Guidor AB) discloses a malleable bioresorbable material used to repair periodontal defects around the tooth. Polylactic acid (PLA) and copolymers of polylactic acid and polyglycolic acid (PGA) are disclosed as usually being very brittle in their pure state. The reference discloses modifying the polymers with plasticizing agents to make them more malleable. The plasticizing agents tend to cause undesirable swelling of the polymers in vivo and to decrease structural stability; however, through careful selection of the plasticizer and polymer and through the use of 10 .mu.m perforations through the material, swelling can be minimized. Such perforations can be created by a laser process as described in PCT Publication WO 92/22336 dated 23 Dec., 1992 to Mathiesen, et al. (Guidor, AB).
U.S. Pat. Nos. 4,844,854 issued Jul. 4, 1989 for "Process for Making a Surgical Device Using Two-phase Compositions," 4,744,365 issued May 17, 1988 and 5,124,103 issued Jun. 23, 1992 for "Two-Phase Compositions for Absorbable Surgical Devices," and 4,839,130 issued Jun. 13, 1989 for "Process of Making an Absorbable Surgical Device," to Kaplan et al. (United States Surgical Corporation) disclose that biodegradable surgical devices of PLA/PGA can be made less brittle by using a two-phase polymer having a continuous lactide-rich phase interpolymerized with a continuous glycolide-rich phase or a continuous lactide-rich phase having dispersed throughout it discrete particles of a glycolide-rich phase. The material is then annealed to raise the temperature at which the material can be distorted. Malleability is thus disclosed as an undesirable property.
As discussed above, it is desirable that a biodegradable implant designed for tissue ingrowth have mechanical properties similar to those of the tissue into which it is placed. Typically accepted values for the elasticity (Young's modulus) of cartilage are less than 1 MPa (Brown and Singerman (1986), J.Biomech. 19(8):597-605). Poisson's ratio (measuring the tendency of the material to distort sideways when pressed upon from the top) values for cartilage are low, no more than about 0.3 (Frank Linde (April, 1994) Danish Med. Bull. Volume 4, No. 2).
It is thus apparent that providing a biodegradable polymeric material with a low Poisson's ratio, which is also hand-shapable at body temperature for use as an implant for promotion of cartilage, bone and other tissue ingrowth is not a trivial problem. A simple malleable substance having a high Poisson's ratio would not be suitable.
Porous biodegradable polymeric implant materials for use as scaffolds for cell ingrowth have previously been limited by the difficulty in achieving uniform porosity in implants of a size larger than a few millimeters in any dimension. Molding such materials in closed molds in vacuum ovens has resulted in the formation of scattered large bubbles and thin spots. Cell ingrowth is best encouraged in a material of uniform porosity. Thus, there is a need for a method of making a biodegradable implant material larger than a few millimeters in cross section with pores of uniform size and distribution.
In traditional bone graft procedures, autogenous cancellous bone from a source such as the iliac crest is often used for filling bone defects. Allogenic bank bone has been advocated as an alternative to autogenous bone, but the effectiveness of the graft is often compromised by nonunions, fatigue fractures, and both clinical and histological evidence of resorption of the graft. Further, allogenic bank bone is often in short supply and may carry disease factors. A major disadvantage in using autogenous bone from the patient's iliac crest is that taking this material is an extremely painful procedure (patients undergoing spine fusions tend to complain more about iliac crest pain than spine pain). Sterile materials useful as substitutes for autogenous and allogenic bone are therefore desirable, especially sterile biodegradable materials serving as scaffolds for in-growing bone.
Biodegradable implant materials for use in healing bone defects include particulate materials such as those described in Ashman, et al. U.S. Pat. Nos. 4,535,485 issued Aug. 20, 1985 for "Polymeric Acrylic Prosthesis" and 4,547,390 for "Process of Making Implantable Prosthesis Material of Modified Polymeric Acrylic (PMMA) Beads Coated with PHEMA and Barium Sulfate." These materials bond together inside the defect to form a porous mass. This implant material is not disclosed to have the mechanical properties of bone. It is desirable in promoting tissue ingrowth to provide an implant material with properties similar to those of the tissue in question insofar as possible to provide a microenvironment such that cells growing into the implant will find conditions as close as possible to the natural conditions for which they were designed. This invention provides such particulate materials.